Modern history of hybrid Fibre/Metal materials starts on the early eighties: Delft University of Technology introduced ARALL (an Aramid/Aluminium Fibre Metal Laminate—FML-) in 1981. Later, Delft-TU together with Fokker Aerospace developed GLARE (Glass Fibre/Aluminium FML), solving structural limitations that prevented a wide use of ARALL. This has been shown with the introduction of GLARE as replacement of monolithic aluminium on some parts of the largest passenger aircraft program, the A380, in 2001.
For other applications, different combinations of materials could be more suitable. In this context, Boeing research programs dealing with Titanium/CFRP (Carbon Fibre Reinforced Plastic) FML during the development of the High Speed Civil Transport (HSCT) is an example: sandwich material made of IM7/PIXA (thermoplastic resin) core with titanium facesheets as high temperature and fatigue resistant material. Also to be applied on the same HSCT program, The Boeing Company obtained in 1999 the patent U.S. Pat. No. 5,866,272 (Titanium-Polymer Hybrid Laminates), where a symmetric hybrid laminate consisting of a central honeycomb core with a number of titanium foils and carbon layers is described.
In parallel, NASA-Langley Research Center performed investigations, also to be applied to a HSCT aircraft, with FML consisting of IM7 carbon tapes (polyamide thermoset resin) with Ti-15V-3Cr-3Al-3Sn foils. Different titanium surface treatments were examined (Report Ref. NASA-98-21 amas-wsj).
As example of a different application, the German Aerospace Centre (DLR—Deutches zentrum fur Luft und Raumfahrt) proposed in 2003 a concept for the transition of a full-CFRP part to a hybrid Ti/CFRP one by progressively replacing most of the carbon tapes by titanium foils in a certain patented manner called “Lamella Coupling System” (WO00/56541, 28th Sep. 2000). The reinforced region results in having Ti foils combined with some of the original 0° plies. Prepregs used were Ciba 6376C-HTA/HTS mixed with Ti-6Al-4V alloy sheets.
A common issue with all fibre/metal materials is the required surface treatment to be applied to the metal to ensure a proper bonding to the composite or to the adhesive layer if exists. With new regulations, old and complex pollutant methods containing high levels of Volatile Organic Compounds (VOC) are no longer allowed. After a joint program involving the U.S. Air Force, the U.S. Navy, the U.S. Army and U.S. Industry Department under a SERDP (Strategic Environmental Research and Development Program) Project numbered PP-1113, the Boeing Company selected the so-called Boegel-EPII as the most promising low-VOC surface treatment technology for titanium bonding. This product is commercially available as AC™-130, from Advanced Chemistry & Technology, Garden Grove, Calif.
Hybrid materials have been used in butt-joints of composite components. For instance GB 1,081,154 published on Aug. 31, 1967 discloses a butt-joint between adjacent ends of reinforced laminated synthetic resin sheet material, such as the ends of a sheet bent to the shape of a cylinder to produce a synthetic-resin silo, in which metal strips are embedded in the adjacent ends and are interconnected.
However hybrid materials have not been used as local reinforcement in heavy loaded lap joints on aircraft primary structures made of composite materials. In this respect a common practice to ensure proper load transfer on current aircraft primary composite structures' heavy loaded joints is to thicken the laminate with additional plies until the required strength is achieved. With this, thickness of the joint can be significantly increased, as thus the weight will do. Examples of heavy loaded joints of typical carbon fibre components could be mechanically fastened joints of spars to skin lifting surfaces, mid-box joints, joints of composite fuselage sections or fitting attachments.
The present invention is intended to solve this drawback.